There are services that users can use and participate in via a network, such as chat and multiplayer games. Such services offer a plurality of sessions each possibly participated in by different users like chat rooms. These sessions provide their services independently of one another. Each user participates in one of a plurality of sessions and uses the service provided by the session.
A user participating in a session may want another user such as his/her friend to take part in the same session. Service providers may also wish to invite specific or indefinite users to participate in a service they provide. Such a service may involve offering the function of inviting users to the session by sending out a session invitation message, for example.